syzbot reported a LOCKDEP splat [1] in rt6_age_examine_exception()

rt6_age_examine_exception() is called while rt6_exception_lock is held.
This lock is the lower one in the lock hierarchy, thus we can not
call dst_neigh_lookup() function, as it can fallback to neigh_create()

We should instead do a pure RCU lookup. As a bonus we avoid
a pair of atomic operations on neigh refcount.

[1]

WARNING: possible circular locking dependency detected
4.16.0-rc4+ #277 Not tainted

syz-executor7/4015 is trying to acquire lock:
 (&ndev->lock){++--}, at: [<00000000416dce19>] __ipv6_dev_mc_dec+0x45/0x350 net/ipv6/mcast.c:928

but task is already holding lock:
 (&tbl->lock){++-.}, at: [<00000000b5cb1d65>] neigh_ifdown+0x3d/0x250 net/core/neighbour.c:292

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (&tbl->lock){++-.}:
       __raw_write_lock_bh include/linux/rwlock_api_smp.h:203 [inline]
       _raw_write_lock_bh+0x31/0x40 kernel/locking/spinlock.c:312
       __neigh_create+0x87e/0x1d90 net/core/neighbour.c:528
       neigh_create include/net/neighbour.h:315 [inline]
       ip6_neigh_lookup+0x9a7/0xba0 net/ipv6/route.c:228
       dst_neigh_lookup include/net/dst.h:405 [inline]
       rt6_age_examine_exception net/ipv6/route.c:1609 [inline]
       rt6_age_exceptions+0x381/0x660 net/ipv6/route.c:1645
       fib6_age+0xfb/0x140 net/ipv6/ip6_fib.c:2033
       fib6_clean_node+0x389/0x580 net/ipv6/ip6_fib.c:1919
       fib6_walk_continue+0x46c/0x8a0 net/ipv6/ip6_fib.c:1845
       fib6_walk+0x91/0xf0 net/ipv6/ip6_fib.c:1893
       fib6_clean_tree+0x1e6/0x340 net/ipv6/ip6_fib.c:1970
       __fib6_clean_all+0x1f4/0x3a0 net/ipv6/ip6_fib.c:1986
       fib6_clean_all net/ipv6/ip6_fib.c:1997 [inline]
       fib6_run_gc+0x16b/0x3c0 net/ipv6/ip6_fib.c:2053
       ndisc_netdev_event+0x3c2/0x4a0 net/ipv6/ndisc.c:1781
       notifier_call_chain+0x136/0x2c0 kernel/notifier.c:93
       __raw_notifier_call_chain kernel/notifier.c:394 [inline]
       raw_notifier_call_chain+0x2d/0x40 kernel/notifier.c:401
       call_netdevice_notifiers_info+0x32/0x70 net/core/dev.c:1707
       call_netdevice_notifiers net/core/dev.c:1725 [inline]
       __dev_notify_flags+0x262/0x430 net/core/dev.c:6960
       dev_change_flags+0xf5/0x140 net/core/dev.c:6994
       devinet_ioctl+0x126a/0x1ac0 net/ipv4/devinet.c:1080
       inet_ioctl+0x184/0x310 net/ipv4/af_inet.c:919
       sock_do_ioctl+0xef/0x390 net/socket.c:957
       sock_ioctl+0x36b/0x610 net/socket.c:1081
       vfs_ioctl fs/ioctl.c:46 [inline]
       do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
       SYSC_ioctl fs/ioctl.c:701 [inline]
       SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
       do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

-> #2 (rt6_exception_lock){+.-.}:
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:135 [inline]
       _raw_spin_lock_bh+0x31/0x40 kernel/locking/spinlock.c:168
       spin_lock_bh include/linux/spinlock.h:315 [inline]
       rt6_flush_exceptions+0x21/0x210 net/ipv6/route.c:1367
       fib6_del_route net/ipv6/ip6_fib.c:1677 [inline]
       fib6_del+0x624/0x12c0 net/ipv6/ip6_fib.c:1761
       __ip6_del_rt+0xc7/0x120 net/ipv6/route.c:2980
       ip6_del_rt+0x132/0x1a0 net/ipv6/route.c:2993
       __ipv6_dev_ac_dec+0x3b1/0x600 net/ipv6/anycast.c:332
       ipv6_dev_ac_dec net/ipv6/anycast.c:345 [inline]
       ipv6_sock_ac_close+0x2b4/0x3e0 net/ipv6/anycast.c:200
       inet6_release+0x48/0x70 net/ipv6/af_inet6.c:433
       sock_release+0x8d/0x1e0 net/socket.c:594
       sock_close+0x16/0x20 net/socket.c:1149
       __fput+0x327/0x7e0 fs/file_table.c:209
       ____fput+0x15/0x20 fs/file_table.c:243
       task_work_run+0x199/0x270 kernel/task_work.c:113
       exit_task_work include/linux/task_work.h:22 [inline]
       do_exit+0x9bb/0x1ad0 kernel/exit.c:865
       do_group_exit+0x149/0x400 kernel/exit.c:968
       get_signal+0x73a/0x16d0 kernel/signal.c:2469
       do_signal+0x90/0x1e90 arch/x86/kernel/signal.c:809
       exit_to_usermode_loop+0x258/0x2f0 arch/x86/entry/common.c:162
       prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
       syscall_return_slowpath arch/x86/entry/common.c:265 [inline]
       do_syscall_64+0x6ec/0x940 arch/x86/entry/common.c:292
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

-> #1 (&(&tb->tb6_lock)->rlock){+.-.}:
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:135 [inline]
       _raw_spin_lock_bh+0x31/0x40 kernel/locking/spinlock.c:168
       spin_lock_bh include/linux/spinlock.h:315 [inline]
       __ip6_ins_rt+0x56/0x90 net/ipv6/route.c:1007
       ip6_route_add+0x141/0x190 net/ipv6/route.c:2955
       addrconf_prefix_route+0x44f/0x620 net/ipv6/addrconf.c:2359
       fixup_permanent_addr net/ipv6/addrconf.c:3368 [inline]
       addrconf_permanent_addr net/ipv6/addrconf.c:3391 [inline]
       addrconf_notify+0x1ad2/0x2310 net/ipv6/addrconf.c:3460
       notifier_call_chain+0x136/0x2c0 kernel/notifier.c:93
       __raw_notifier_call_chain kernel/notifier.c:394 [inline]
       raw_notifier_call_chain+0x2d/0x40 kernel/notifier.c:401
       call_netdevice_notifiers_info+0x32/0x70 net/core/dev.c:1707
       call_netdevice_notifiers net/core/dev.c:1725 [inline]
       __dev_notify_flags+0x15d/0x430 net/core/dev.c:6958
       dev_change_flags+0xf5/0x140 net/core/dev.c:6994
       do_setlink+0xa22/0x3bb0 net/core/rtnetlink.c:2357
       rtnl_newlink+0xf37/0x1a50 net/core/rtnetlink.c:2965
       rtnetlink_rcv_msg+0x57f/0xb10 net/core/rtnetlink.c:4641
       netlink_rcv_skb+0x14b/0x380 net/netlink/af_netlink.c:2444
       rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4659
       netlink_unicast_kernel net/netlink/af_netlink.c:1308 [inline]
       netlink_unicast+0x4c4/0x6b0 net/netlink/af_netlink.c:1334
       netlink_sendmsg+0xa4a/0xe60 net/netlink/af_netlink.c:1897
       sock_sendmsg_nosec net/socket.c:629 [inline]
       sock_sendmsg+0xca/0x110 net/socket.c:639
       ___sys_sendmsg+0x767/0x8b0 net/socket.c:2047
       __sys_sendmsg+0xe5/0x210 net/socket.c:2081
       SYSC_sendmsg net/socket.c:2092 [inline]
       SyS_sendmsg+0x2d/0x50 net/socket.c:2088
       do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

-> #0 (&ndev->lock){++--}:
       lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
       __raw_write_lock_bh include/linux/rwlock_api_smp.h:203 [inline]
       _raw_write_lock_bh+0x31/0x40 kernel/locking/spinlock.c:312
       __ipv6_dev_mc_dec+0x45/0x350 net/ipv6/mcast.c:928
       ipv6_dev_mc_dec+0x110/0x1f0 net/ipv6/mcast.c:961
       pndisc_destructor+0x21a/0x340 net/ipv6/ndisc.c:392
       pneigh_ifdown net/core/neighbour.c:695 [inline]
       neigh_ifdown+0x149/0x250 net/core/neighbour.c:294
       rt6_disable_ip+0x537/0x700 net/ipv6/route.c:3874
       addrconf_ifdown+0x14b/0x14f0 net/ipv6/addrconf.c:3633
       addrconf_notify+0x5f8/0x2310 net/ipv6/addrconf.c:3557
       notifier_call_chain+0x136/0x2c0 kernel/notifier.c:93
       __raw_notifier_call_chain kernel/notifier.c:394 [inline]
       raw_notifier_call_chain+0x2d/0x40 kernel/notifier.c:401
       call_netdevice_notifiers_info+0x32/0x70 net/core/dev.c:1707
       call_netdevice_notifiers net/core/dev.c:1725 [inline]
       __dev_notify_flags+0x262/0x430 net/core/dev.c:6960
       dev_change_flags+0xf5/0x140 net/core/dev.c:6994
       devinet_ioctl+0x126a/0x1ac0 net/ipv4/devinet.c:1080
       inet_ioctl+0x184/0x310 net/ipv4/af_inet.c:919
       packet_ioctl+0x1ff/0x310 net/packet/af_packet.c:4066
       sock_do_ioctl+0xef/0x390 net/socket.c:957
       sock_ioctl+0x36b/0x610 net/socket.c:1081
       vfs_ioctl fs/ioctl.c:46 [inline]
       do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
       SYSC_ioctl fs/ioctl.c:701 [inline]
       SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
       do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

other info that might help us debug this:

Chain exists of:
  &ndev->lock --> rt6_exception_lock --> &tbl->lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&tbl->lock);
                               lock(rt6_exception_lock);
                               lock(&tbl->lock);
  lock(&ndev->lock);

 *** DEADLOCK ***

2 locks held by syz-executor7/4015:
 #0:  (rtnl_mutex){+.+.}, at: [<00000000a2f16daa>] rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
 #1:  (&tbl->lock){++-.}, at: [<00000000b5cb1d65>] neigh_ifdown+0x3d/0x250 net/core/neighbour.c:292

stack backtrace:
CPU: 0 PID: 4015 Comm: syz-executor7 Not tainted 4.16.0-rc4+ #277
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x24d lib/dump_stack.c:53
 print_circular_bug.isra.38+0x2cd/0x2dc kernel/locking/lockdep.c:1223
 check_prev_add kernel/locking/lockdep.c:1863 [inline]
 check_prevs_add kernel/locking/lockdep.c:1976 [inline]
 validate_chain kernel/locking/lockdep.c:2417 [inline]
 __lock_acquire+0x30a8/0x3e00 kernel/locking/lockdep.c:3431
 lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3920
 __raw_write_lock_bh include/linux/rwlock_api_smp.h:203 [inline]
 _raw_write_lock_bh+0x31/0x40 kernel/locking/spinlock.c:312
 __ipv6_dev_mc_dec+0x45/0x350 net/ipv6/mcast.c:928
 ipv6_dev_mc_dec+0x110/0x1f0 net/ipv6/mcast.c:961
 pndisc_destructor+0x21a/0x340 net/ipv6/ndisc.c:392
 pneigh_ifdown net/core/neighbour.c:695 [inline]
 neigh_ifdown+0x149/0x250 net/core/neighbour.c:294
 rt6_disable_ip+0x537/0x700 net/ipv6/route.c:3874
 addrconf_ifdown+0x14b/0x14f0 net/ipv6/addrconf.c:3633
 addrconf_notify+0x5f8/0x2310 net/ipv6/addrconf.c:3557
 notifier_call_chain+0x136/0x2c0 kernel/notifier.c:93
 __raw_notifier_call_chain kernel/notifier.c:394 [inline]
 raw_notifier_call_chain+0x2d/0x40 kernel/notifier.c:401
 call_netdevice_notifiers_info+0x32/0x70 net/core/dev.c:1707
 call_netdevice_notifiers net/core/dev.c:1725 [inline]
 __dev_notify_flags+0x262/0x430 net/core/dev.c:6960
 dev_change_flags+0xf5/0x140 net/core/dev.c:6994
 devinet_ioctl+0x126a/0x1ac0 net/ipv4/devinet.c:1080
 inet_ioctl+0x184/0x310 net/ipv4/af_inet.c:919
 packet_ioctl+0x1ff/0x310 net/packet/af_packet.c:4066
 sock_do_ioctl+0xef/0x390 net/socket.c:957
 sock_ioctl+0x36b/0x610 net/socket.c:1081
 vfs_ioctl fs/ioctl.c:46 [inline]
 do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Fixes: c757faa8 ("ipv6: prepare fib6_age() for exception table")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Wei Wang <weiwan@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Acked-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>